The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel, to maintain the patency thereof. Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the results of the procedure and reduce the possibility of restenosis.
Stents are generally tubular shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as a coronary artery. Stents are usually delivered in a radially compressed condition to the target site and then deployed at that location into a radially expanded condition to support the wall of the vessel and help maintain it dilated. They are particularly suitable for urging a segment of a dissected arterial lining radially outwardly in a lumen to maintain a fluid passageway there through.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted in a compressed state for deployment into a body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the wall of the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed, for example, from shape memory metals such as nickel-titanum (NiTi) alloys, which will respond to elevated temperature or the like to expand from a radially compressed state when the stent is advanced out of the distal end of the delivery catheter into the blood vessel. Such stents manufactured from expandable heat sensitive materials allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent. Other self-expanding stents may use stress-induced martensite (SIM) alloys to allow the stent to move between contracted and expanded positions.
Typical stent delivery systems for implanting expandable stents at the target site generally include a dilatation catheter having an inflatable balloon or other expandable means mounted at the distal end thereof. The expandable stent is radially compressed onto the balloon for delivery within a body lumen. Some prior art stent delivery systems for implanting balloon expandable stents utilize an outer delivery sheath that is initially placed over the compressed stent prior to delivery. A delivery sheath is sometimes used to prevent the compressed stent from moving axially along the balloon portion of the dilatation catheter while being advanced within the patient""s vasculature. Once the catheter is in place, the physician can retract the outer sheath to expose the stent and expandable balloon. The physician can then inflate the balloon portion of the dilatation catheter to cause the compressed stent to expand radially to a larger diameter to be left in place within the artery at the target site.
In the case of implanting self-expanding stents at the target site, typical delivery systems include an inner lumen upon which the compressed or collapsed stent is mounted and an outer restraining sheath that is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner lumen to xe2x80x9cuncoverxe2x80x9d the compressed stent, allowing the stent to move to its expanded condition. Some delivery systems utilize a xe2x80x9cpush-pullxe2x80x9d technique in which the outer sheath is retracted while the inner tubing is pushed forward. Still other systems use an actuating wire that is attached to the outer sheath. When the actuating wire is pulled to retract the outer sheath and deploy the stent, the inner tubing must remain stationary, thereby preventing the stent from moving axially within the body lumen.
Prior art stent delivery systems are benefitted by the function of the delivery sheath in preventing the collapsed stent from moving axially along the inner lumen of the delivery catheter while being advanced within the patient""s vasculature. In addition, sometimes the stent cannot be deployed for a variety of reasons, so it must be able to be pulled back into the guiding catheter without being xe2x80x9cstripped offxe2x80x9d of the dilatation or delivery catheter. Further, despite the care given during placement, stents can become dislodged from the delivery system. The consequences of losing a stent range from embarrassment to a life threatening situation requiring immediate surgery. The use of a delivery sheath can help alleviate such problems.
The delivery sheath also helps prevent the stent from abrading the body lumen wall as the stent is being manipulated into the target area. With no delivery sheath, the struts of the stent would be exposed to the walls of the patient""s vasculature and could possibly cause trauma to the walls or cause pieces of plaque to break from the stenosis. Abrasive forces in the area of the stenosis are not desirable due to the possible formation of embolic debris that would be released into the patient""s blood stream. Such debris could possibly occlude smaller blood vessels leading to vital organs such as the brain. Thus, for a variety of reasons, the outer sheath remains in place over the compressed stent until the physician has manipulated the catheter into the proper location within the patient""s vasculature. Once in position, the physician can retract the outer sheath to expose the stent and allow it to safely expand within the body lumen at the target site.
It follows that it is beneficial for the delivery sheath to have a low profile with no obtrusions and that the sheath be made of a low-friction, flexible material. Such construction would facilitate the insertion of the stent delivery system into small inner diameter guide catheters and body lumens and would minimize trauma to the lumens as the delivery system is being maneuvered into tight, difficult-to-reach areas in the patient""s vasculature.
When used with a self-expanding stent delivery system, the delivery sheath must serve an additional purpose of resisting the radial force being applied to the sheath by the stent as it is held in its collapsed condition. In some self-expanding stent designs, the radial force applied by a collapsed stent can be quite substantial. As a result, the delivery sheath must have sufficient strength to support the collapsed stent against expansion. Additionally, since a stent delivery system may be placed in storage for a considerable length of time, the sheath must be capable of restraining the stent during that period. The prolonged exposure of the sheath to an expansive force can ultimately deform the sheath (referred to as creep), which can render the delivery system useless for implantation in a patient.
The path to the deployment site within a patient""s body lumen may be relatively tortuous, involving navigation through various curves and turns of the patient""s vasculature, which requires longitudinal flexibility in order to accommodate the turns without inflicting trauma to the walls of the lumen. In the past, this requirement for flexibility dictated a relatively thin wall sheath and placed the constraint on the designer trading off sheath strength for flexibility. There thus exists a need for a sheath which affords sufficient radial strength to maintain the self-expanding stent compressed during deployment, but yet has sufficient flexibility along the longitudinal axis to accommodate navigation through various curves and turns in the body lumen.
Moreover, in some prior art self-expanding stent delivery systems, the frictional force of the sheath against the stent can cause the stent to somewhat contract axially against the resiliency of the undulations of the stent structure, thus causing the stent to store energy. Such stored energy can be released as the sheath is fully retracted off the length of the stent, causing the stent to move or xe2x80x9cjumpxe2x80x9d distally from the end of the sheath, thereby shifting it from the desired position and resulting in inaccurate placement in the body vessel. The amount of energy stored is a function of the flexibility of both the stent and the sheath and frictional forces generated between them. Therefore, it is important that the delivery sheath allow the self-expanding stent to slide relatively freely relative to the sheath as the sheath is being retracted in order to achieve smooth and accurate deployment of the stent.
What has been needed and heretofore unavailable is a delivery sheath to be utilized in conjunction with a stent delivery system that has increased flexibility and reduced frictional contact with the stent, while still effectively retaining the stent and preventing the stent from contacting and injuring the body lumen wall during manipulation of the delivery system into the treatment area. The present invention disclosed herein satisfies these and other needs.
The present invention is directed to a delivery sheath for use in conjunction with a stent delivery system which has increased flexibility and reduced frictional contact with the stent. The delivery sheath is uniquely composed of a series of longitudinally spaced apart retainer rings interconnected by flexible links. This configuration provides for sections of reduced cross-sectional area along the length of the sheath, thus increasing flexibility in those sections and overall in the sheath.
The reduction of the cross-sectional area of the delivery sheath of the present invention achieved by incorporating the flexible links into the sheath structure can also serve to effectively reduce the area of contact between the inside surface of the sheath and the outside surface of the compressed stent. This reduction in contact area serves to reduce the frictional force generated between the sheath and the stent as the sheath is retracted relative to the stent during deployment. Such reduction in friction is particularly advantageous for delivery of self-expanding stents, where frictional energy can be stored axially in the stent as the sheath is retracted, causing the stent to move or xe2x80x9cjumpxe2x80x9d forward during deployment, thus possibly resulting in inaccurate stent placement.
One conventional stent delivery system with which the sheath of the present invention cooperates includes an elongated, flexible catheter body comprised of an inner tubular member that extends within an outer tubular member in a coaxial arrangement. In the case of balloon expandable stents, the inner tubular member includes an inflatable balloon or other expansion means at its distal end. The outer tubular member has the delivery sheath mounted at its distal end to cover the stent as the delivery system is advanced within the patient""s vasculature and also, for self-expanding stents, to retain the stent in a radially compressed delivery position on the inner tubular member until deployment. The outer tubular member and delivery sheath are slidably retractable relative to the inner tubular member in order to deploy the stent to its expanded condition within the body lumen.
Therefore, the present invention provides a delivery sheath with increased flexibility and reduced friction to cooperate with the stent delivery system to more easily negotiate tortuous anatomy within a patient""s vasculature and more accurately deploy the stent at the target site. Ultimately, the benefits of the present invention is safer and more effective stent delivery. Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features of the invention.